uprobes: Decouple return_instance list traversal and freeing

free_ret_instance() has two unrelated responsibilities: actually
cleaning up return_instance's resources and freeing memory, and also
helping with utask->return_instances list traversal by returning the
next alive pointer.

There is no reason why these two aspects have to be mixed together, so
turn free_ret_instance() into void-returning function and make callers
do list traversal on their own.

We'll use this simplification in the next patch that will guarantee that
to-be-freed return_instance isn't reachable from utask->return_instances
list.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20241206002417.3295533-3-andrii@kernel.org